ISPconfig on kvm

Sorry for bothering you - but about one year of googling along with trial-and-error did not solve the topic.

I am running ISPconfig3, based on Debian lenny, as a kvm-guest on a proxmox-server. The setup caused no problems whatsoever, but from 4 GB of memory assigend to the guest, only 885 MB are recognized and used.

And this causes mysql to produce a repeating failure:

ERROR 1135 (00000): Can't create a new thread (errno 12); if you are not out of available memory, you can consult the manual for a possible OS-dependent bug.